1. Field of the Invention
The present invention relates to a cylindrical alkaline storage battery such as a nickel-hydrogen battery, a nickel-cadmium battery, a nickel-zinc battery in the like, and more particularly to an improvement of a conductive connection between a current collector and an electrode in the form of an active material retention substrate impregnated with an active material.
2. Discussion of the Prior Art
An electrode for use in a conventional alkaline storage battery such as a nickel-cadmium battery, a nickel-hydrogen battery, a nickel-zinc battery or the like is in the form of a sintered type electrode fabricated by the steps of sintering nickel powder on a perforated core substrate such as a punched sheet metal to produce a sintered base plate. impregnating the sintered base plate with solution of nickel and cadmium salts and subjecting the base plate to alkali treatment for activation. In case a sintered base plate of high porosity was used for fabrication of the sintered type electrode, the mechanical strength of the electrode would be weakened. For this reason, a sintered base plate of about 80% porosity is used for fabrication of the electrode in practical use. In addition, use of the perforated core substrate results in a decrease of impregnation density of the active material and difficulty for fabrication of an electrode of high energy density. Since the pore of the sintered base plate is less than 10 .mu.m, to the impregnation process of the active material must be resealed by a solution impregnation method or an electrodepositive impregnation method. This results in an increase in the manufacturing cost of the batteries.
To avoid the problems described above, a non-sintered type electrode has been used, which is fabricated by directly impregnating paste of an active material into a porous metal substrate or an active material retention substrate of three dimensionally meshed structure such as a sintered substrate of metal fiber, foam nickel or nickel sponge. Since the porosity of the porous metal substrate of three dimensionally meshed structure is about 95%, the porous metal substrate can be impregnated with the active material of high density for manufacturing a battery of large capacity in a simple manner without a treatment for activation.
As the non-sintered type electrode of this kind does not include any core substrate, various methods have been proposed for conductive connection of the battery terminal with the electrode in the form of the porous metal substrate impregnated with the active material. For example, disclosed in Japanese Patent Laid-open Publication No. 61-218067 is a manufacturing method of an electrode the retention substrate of which is in the form of a sintered felt-like substrate of metal fiber. During the manufacturing process of the electrode, a felt-like plate of metal fiber is integrally formed with a conductive ancillary substrate such as a meshed substrate, a punched sheet metal, a wire material or a flat plate to enhance the mechanical strength of the felt-like plate of metal fiber and the current collectivity of the electrode.
However, as the sintered substrate of metal fiber is made of fine metal fibers of about 10 .mu.m in diameter bundled in a longitudinal direction of the electrode, the fine metal fibers are disconnected when the sintered substrate of metal fiber is spirally rolled up through a separator after being coated with the active material. As a result, the positive and negative electrodes are electrically connected by fragments of the metal fibers that pierce the separator, resulting in the occurrence of a short circuit in the battery.
In the electrode fabricated by using the substrate of foam nickel, the foam nickel itself is prevented from disconnection even when the electrode is spirally rolled up through the separator after being impregnated with the active material. In this case, the active material of the electrode is peeled off to expose the foam nickel, and a current collector tab is welded to the exposed portion of the foam nickel for current collection. In such construction of the electrode, however, voltage drop will occur at the current collector tab when a large amount of current is discharged.
Disclosed in Japanese Patent Laid-open Publication No. 62-139251 is a nickel-cadmium battery fabricated by the steps of compressing one end portion of an electrode substrate made of foam nickel in its width direction to form a dense layer without impregnation of any active material and welding the dense layer to a circular lead plate placed perpendicularly to the electrode surface. In the electrode of the nickel-cadmium battery, the electrode substrate made of foam nickel itself is prevented from disconnection even when the electrode is spirally rolled up through a separator, and the current collectivity at the compressed one end of the electrode substrate welded to the circular lead plate is enhanced. However, as the dense layer formed by compressing the one end portion of the electrode substrate in its width direction is inferior in elasticity, the dense layer is partly broken when the electrode is spirally rolled up through the separator. As a result, a short circuit in the battery will occur due to burrs of the dense layer that pierce the separator. If the dense layer is mixed with an elastic portion of the electrode, it becomes difficult to spirally roll up positive and negative electrodes under uniform pressure.
It has been also proposed to form one end portion of the substrate of foam nickel without impregnation of any active material thereby to fabricate an electrode by welding a ribbon-like sheet metal on the one end portion of the substrate. However, when the electrode is spirally rolled up with an opposed electrode through a separator, a portion -of the sheet metal is folded and brought into contact with the opposed electrode to cause a short circuit in the battery.